I've got a number of videos on youtube all with around 10 keyword tags per video. All of the tags are definitely relevant to the video and I include keywords that are common and relate to the playlist (4 out of the 10 keywords).

I've heard some people say recently though it's better to go after maybe just one main tag?

What are your thoughts as I was wanting to do maybe do some experimenting?

However, seems like a double edged sword...because if you have more related tags your videos can also show up as related..